bug#34038: 26.1; set-window-start sometimes fails to set window start

[Eli Zaretskii]

> From: Markus Triska <triska@metalevel.at>
> Cc: rudalics@gmx.at,  acm@muc.de,  34038@debbugs.gnu.org
> Date: Fri, 29 Mar 2019 08:16:25 +0100
> 
> The key issue seems to be that point must be set correctly: If point is
> at a position that is known to be displayable with a given window-start,
> then if point is again set to that position, set-window-start will also
> reliably set window-start to that start. Is this correct?

Yes.  In fact, that's how set-window-start was intended to work to
begin with: for applications that put point at certain location, and
also want to control where the window display will start on next
redisplay.  Scrolling commands behave that way, for example.

> Personally, I would appreciate some comment regarding such a guarantee,
> or the assumptions, in the documentation of set-window-start.

OK, I will add something to that effect, thanks for pointing this out.